Apparatus for inductively heating metal strip



April' 11, 1967 H. GEISEL ETAL APPARATUS FOR INDUCTIVELY HEATING METAL STRIP Filed Aug. 17, 1964 5 Sheets-Sheet 1 Fig. 3

April 11, 1967 `H. GEISEL ETAL 3,313,907

APPARATUS FOR INDUCTIVELY HEATING METAL STRIP Filed Aug. 17, 1964 5 Sheets-Sheet :3

l l l l h. vvvv April 1l, 1967 H. GEISEI. ETAL APPARATUS FIOR INDUCTIVELY HEATING METAL STRIP 5 Sheets-Sheet 5 Filed Aug. 17, 1964 Fig. 4

United States Patent O 3,313,907 APPARATUS FOR INDUCTIVELY HEATING l METAL STRIP Herbert Geisel, Remscheid, Luttringhausen, Hans Heinz The invention relates to equipment for inductivelyheating metal strip, particularly for fusing coatings on to such strip, such as electrolytically deposited layers of tin or for drying coatings of varnishA thereon. In machines used for such purposes several high or medium frequency coils are provided in sequential groups and the strip is conducted therethrough over deflecting rollers. Frequently the arrangement is such` that the strip travels up and down over the rollers.

In conventional equipment certain diiiiculties arise which are due to the fact that the temperature rise in the strip cannot be easily distributed uniformly across the wi-dth of the strip. This is a particular nuisance when fusing electrolytically ldeposited layers of tin on to the strip.

For .performing such work equipment is known such as that schematically illustrated in FIG. l of the accompanying drawings. The thin metal strip, as indicated at 1, runs through a tank 2 containing a heated liquid. From this tank the strip travels upwards over a deecting roller 3 and then downwards through a group of coils 4, which in the illustrated example comprises nine coils. The strip is then taken over a roller 3 upwards again to a further deecting roller 3 through a second set of coils 6 comprising say nine or ten coils. The strip is pre-heated in the two groups of coils 4 and 6 and reaches the melting temperature of the tin upon ldescending through a final group 7 of six coils.

The problem is to ensure that the melting temperature is reached at the same time at all .points across the entire width of the strip shortly before the strip is plunged below the surface S of a quenching liquid in a further tank 9 in which the strip is again re-deected at 10.

It will be understood from the schematic illustration on FIG. 2 of the accompanying drawings that the known `arrangement achieves the desired uniform distribution very inadequately. The temperature ldistribution in the strip 11, as it passes through the final three coils 12, 13 and 14 of group 7 is as indicated by line 15, that is to say a uniform temperature slightly exceeding 230 C. builds up across a substantial part of the strip, as required for fusing the tin. In practice this line is clearly visible. Before the tin melts `the strip has a matt appearance, whereas the par-ts upon which fusion has already occurred are lustrous and they retain this lus-tre even after they have been quenched. However, in the margins 16 of the strip the desired temperature is not reached and fusion does not therefore -take place until later. The fusion boundary line is therefore arched and this line does not straighten out by the time the strip plunges below the surface 8 of the quenching liquid.

The required compensation cannot be brought about by increasing the power or by changing the speed of the strip. The only result would probably be to raise the temperature in region to far above the fusion point of the tin. This would cause the formation of a thick iron-tin intermediate layer which is brittle, and which therefore makes its presence adversely felt when the strip is subjected to further treatments. The willingness of the strip to accept a coating of lacquer is likewise adversely affected. Furthermore, the length of the descending branches 17 of the fusion boundary line is extended, indicating that the marginal zones in which the tin has not properly fused are even larger than they were before. Furthermore, the fusion boundary line must not be allowed to creep too far upwards because it is otherwise possible for fusion to occur in the region of roller 3", in which case faulty surfaces will result. i

EXpedients such as tighter coupling between the coils and the edges of the strip or a modification of the effective surfaces of the coils in this region do not lead to the desired result. It is also impossible to equalise the temperature distribution by providing supplementary coils which merely embrace the strip edges.

However, it has been found -that surprisingly it is possible to solve the problem by providing, according to the invention, in addition to the coils embracing the strip, preferably in a position preceding the iinal group of coils, inductors with line-ar heating conductors on each side margin. They may be located marginally on both surfaces of the strip if measures are taken to ensure that the direction of the respective currents is the same in each case. However, the most economical solution is to provide inductors acting only on one surface of the strip, since it has been found that such an arrangement is entirely adequate.

The present invention is illustrated in FIGS. 3 and 4 of the accompanying drawings. FIG. 3 is a sectional elevation through the linear inductors and the strip and FIG. 4 shows in perspective the traverse of the strip and detector means cooperative with the linear inductors.

Referring to FIG. 3, facing each side margin on one side of the strip 11 is a linear inductor 18. These supplementary inductors 18 may be located either in the direction of travel of the strip after having passed over roller 3 and they will then directly precede the group of coils 7, or alternatively, they may be located as indicated in dotted lines at 18 at a point preceding roller 3".

The inductors 18 which in conventional manner may have conductors provided with laminar yokes 19 must be positioned in relation to the strip edges roughly in the manner indicated in FIG. 3. They will then have the effect of straightening the fusion boundary line an-d of thus ensuring that the tin fuses on to the strip uniformly across the entire width of the strip. Moreover, the proposed arrangement eliminates the risk of undesirable overheating accompanied by a deterioration of the tinned surface anda thickening of the tin-iron intermediate layer.

Since the strip may migrate to and fro on the defiecting rollers and its width may vary, it is advisable to associate the linear inductors with detector devices which automatically adjust the position of the inductors in relation to the strip edges.

FIG. 4 schematically shows how this can be done. The inductors 18 together with detector means 20 are mounted on a frame 21. If the width of the strip varies in the directions indicated by arrow a or if the strip should migrate in the direction of arrow b or c, then the detector 20 will generate signals for the actuation of a hydraulic cylinder 22 to shift the inductors 18 inwards or outwards as required by means of a piston and rod.

The linear inductors for controlling the temperature at the strip edges should preferably be continuously variable. Moreover, they should also be adaptable to varying speeds of travel of the strip and hence to varying throughputs.

What we claim is:

1. Apparatus for raising the temperature of a meta-l strip to a higher, predetermined temperature which is uniform across the width of the strip, comprising:

(a) means for conveying said metal strip generally longitudinally through rst and subsequent heating stages,

(b) said rst heating stage including means for heating the central portion of the moving metal strip, and

(c) said subsequent heating stage including means for heating the longitudinal marginal edges of the strip,

(d) said edge heating means comprising linear heating conductors disposed to face and heat each of the two side margins of the strip.

2. Apparatus according to claim 1, wherein the conductors are disposed so as to face only one surface of the strip.

3. Apparatus according to claim 1, further comprising detector means responsive to the position of the strip edges, and cooperative yWith said linear heating conductors, for automatically adjusting said conductors according to the positions of said edges.

4. A method of fusing a coating on metallic strip, comprising:

(a) rst heating the central portion of the strip to a predetermined temperature by rst heating means,

(b) subsequently heating the longitudinal marginal edges of the strip by linear heating conductors presented marginally along the strip, whereby the strip becomes heated uniformly across the width, and (c) next heating the strip with induction coils to pro- 5 mote uniformity of fusion of the coating.

5. The method according to claim 4 wherein the coating on the metal -strip is tin.

References Cited by the Examiner RICHARD M. WOOD, Primary Examiner. 20 L. H. BENDER, Animar Examiner. 

1. APPARATUS FOR RAISING THE TEMPERATURE OF A METAL STRIP TO A HIGHER, PREDETERMINED TEMPERATURE WHICH IS UNIFORM ACROSS THE WIDTH OF THE STRIP, COMPRISING: (A) MEANS FOR CONVEYING SAID METAL STRIP GENERALLY LONGITUDINALLY THROUGH FIRST AND SUBSEQUENT HEATING STAGES, (B) SAID FIRST HEATING STAGE INCLUDING MEANS FOR HEATING THE CENTRAL PORTION OF THE MOVING METAL STRIP, AND (C) SAID SUBSEQUENT HEATING STAGE INCLUDING MEANS FOR 